Method and apparatus for determining a capacity of a battery

ABSTRACT

Some embodiments of the present invention provide a system that accurately and reliably updates a full charge capacity of a battery. During operation, the system charges the battery from an initial state to a rest point prior to reaching a fully charged state. The system then interrupts the charging process to allow the battery to relax to a resting voltage. Next, the system measures the resting voltage. The system then resumes the charging process toward the fully charged state. The system subsequently estimates the capacity of the battery based on the measured resting voltage and one or more other parameters.

BACKGROUND

1. Field

The present invention generally relates to techniques for charging arechargeable battery. More specifically, the present invention relatesto a method and apparatus for updating the full charge capacity of arechargeable battery during a charging process for the battery.

2. Related Art

Rechargeable lithium-ion batteries are presently used to provide powerin a wide variety of systems, including smartphones, wireless devices,laptop computers, cordless power tools and electric vehicles. The fullcharge capacity of a battery (often referred to as “Q_(max)”) is ameasurement of the maximum chemical capacity of a rechargeable battery.As battery cells age, the full charge capacity of the battery generallydecreases. Hence, measuring and updating the full charge capacity isfundamental to basic battery management, such as determiningstate-of-charge, reserve, use time and battery health.

To accurately estimate the full charge capacity of a battery after ithas aged, existing techniques rely on an assumption that a userdischarges the battery from a high state of charge to a low state ofcharge, with long rest periods at both endpoints of the dischargingprocess, which allows two relaxed-voltage measurements to be taken atthe two endpoints. Unfortunately, these existing techniques often failto update the full charge capacity of a battery as a result of actualuser behavior.

In reality, users typically leave their systems plugged in for a whileafter the systems have been fully charged, which allows an accuraterelaxed measurement to be obtained at the full state of charge. However,it has been observed in the field that users do not typically allowrests at low states of charge. On the contrary, as soon as the batterydischarges down to a low state of charge, the user typically plugs inthe battery and charges it up again. Consequently, the battery does notrest and relax to the point where a measurement can be obtained at thelow state of charge. This user behavior results in infrequent or even acomplete absence of updates of the full charge capacity of a battery.This deficiency, in turn, can cause inaccurate reserve calculations andinaccurate gauging of a battery's state of charge which can lead to dataloss from system brown-outs.

Note that problems that arise from the absence of updates to the fullcharge capacity can become worse for mobile phones because, even whenthe phones are not actively used, they are often configured to runapplications in the background. As a result, some users almost never getfull charge capacity updates because the batteries are not allowed torest at the low states of charge. This means that over the lifetime of amobile phone, while the capacity of the battery has become significantlysmaller, the device might not be able to measure this accurately andwill show an inaccurate state-of-charge to the user.

Hence, what is needed is a method and an apparatus for accurately andreliably updating a full charge capacity of a battery without theabove-described problems.

SUMMARY

The described embodiments provide a system that accurately and reliablyupdates a full charge capacity of a battery. During operation, thesystem charges the battery from an initial state to a rest point priorto reaching a fully charged state. The system then interrupts thecharging process to allow the battery to relax to a resting voltage.Next, the system measures the resting voltage. The system then resumesthe charging process toward the fully charged state. The systemsubsequently estimates the capacity of the battery based on the measuredresting voltage and one or more other parameters.

In some embodiments, prior to charging the battery, the systemdetermines if a state of charge of the battery corresponding to theinitial state is above a state of charge of the battery corresponding tothe rest point. If so, the system charges the battery from the initialstate to the fully charged state without interrupting the chargingprocess.

In some embodiments, the system charges the battery with a constantcurrent during a time period immediately preceding the rest point.

In some embodiments, the system interrupts the charging process byimmediately dropping the charging current to zero.

In some embodiments, the system interrupts the charging process byreducing the charging current to a low level above zero.

In some embodiments, after reaching the fully charged state, the systemallows the battery to relax to a second resting voltage and thenmeasures the second resting voltage corresponding to the fully chargedstate.

In some embodiments, the system determines the capacity of the batteryby computing a first state of charge of the battery corresponding to theresting voltage. The system also computes a second state of charge ofthe battery corresponding to the second resting voltage. The systemadditionally determines a coulomb count between the rest point and thefully charged state. The system subsequently determines the capacity ofthe battery based on the first state of charge, the second state ofcharge, and the coulomb count.

In some embodiments, the system determines the coulomb count between therest point and the fully charged state by initiating a coulomb countingfrom the rest point after resuming the charging process. The system thenconcludes the coulomb counting when the fully charged state is reached.The system can then determine the coulomb count based on the coulombcounting between the rest point and the fully charged state.

In some embodiments, each of the resting voltage and the second restingvoltage is an open circuit voltage (OCV).

In some embodiments, prior to charging the battery, the system selectsthe rest point based at least on one or more of the followingparameters: a time required for the battery to relax at the rest point;an error associated with coulomb counting between the rest point and thefully charged state; a likelihood of a user discharging the batterybelow the rest point; and an open-circuit-voltage measurement accuracyrequired to compute a state of charge associated with the rest point.

In some embodiments, the system selects the rest point by ensuring thatthe time required for the battery to relax at the rest point issignificantly shorter than a time to charge the battery from the initialstate to the fully charged state without interruption.

In some embodiments, the rest point is at or above a 60% state of chargeof the battery.

In some embodiments, the rest point is at or below an 80% state ofcharge of the battery.

One embodiment of the present invention provides a battery with acapacity estimation mechanism. This battery includes a current sensor tomeasure a current for the battery, and a voltage sensor to measure avoltage across terminals of the battery. The capacity estimation processis under the control of a controller, which receives inputs from thevoltage sensor and the current sensor, and generates a capacityestimate. During the capacity estimation process, the controller (1)charges the battery from an initial state to a rest point prior toreaching a fully charged state; (2) interrupts the charging process toallow the battery to relax to a resting voltage; (3) measures theresting voltage; (4) resumes the charging process toward the fullycharged state; and (5) estimates the capacity of the battery based onthe measured resting voltages and the coulomb count between themeasurement points.

Some embodiments described in the present disclosure also provide asystem that accurately evaluates a state of charge of a battery during acharging process. During operation, the system charges the battery froman initial state to a rest point prior to reaching a fully chargedstate. The system then interrupts the charging process to allow thebattery to relax to a resting voltage. Next, the system measures theresting voltage. The system subsequently determines a state of charge ofthe battery at the rest point based on the measured resting voltage.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a coulomb counting based Q_(max) measurementtechnique for a battery in accordance with some embodiments herein.

FIG. 2 presents a flowchart illustrating a process of charging a batterythat includes a controlled Q_(max) update in accordance with someembodiments herein.

FIG. 3 presents a flowchart illustrating a modified charging processwhich uses an inserted rest point for a controlled Q_(max) update inaccordance with some embodiments herein.

FIG. 4 illustrates a charging profile that employs a constant-currentconstant-voltage charging profile in accordance with some embodimentsherein.

FIG. 5 illustrates a charging profile of a controlled Q_(max) updatethat includes a single rest point in accordance with some embodimentsherein.

FIG. 6 illustrates an exemplary cell relaxing profile after a constantcurrent charging process has been applied up to a rest point at a highSOC in accordance with some embodiments herein.

FIG. 7 provides a chart illustrating different state of charge regionswhich may be used to guide the choice of a proper rest point.

FIG. 8 illustrates a rechargeable battery that supports a controlledQ_(max) update during charging in accordance with some embodimentsherein.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing computer-readable media now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium. Furthermore, the methodsand processes described below can be included in hardware modules. Forexample, the hardware modules can include, but are not limited to,application-specific integrated circuit (ASIC) chips, field-programmablegate arrays (FPGAs), and other programmable-logic devices now known orlater developed. When the hardware modules are activated, the hardwaremodules perform the methods and processes included within the hardwaremodules.

TERMINOLOGY

Throughout the specification, the following terms have the meaningsprovided herein, unless the context clearly dictates otherwise. The term“battery” generally means a rechargeable battery which includes a cellpack (with one or more cells). Hence, a term such as “full chargecapacity of a battery” means “full charge capacity of the cell packwithin the battery;” “charging a battery” means “charging the cell packwithin the battery;” and “allowing the battery to relax” means “allowingthe cell pack within the battery to relax.” Furthermore, terms “rested”and “relaxed” are used interchangeably to refer to a state of a batterywherein the current in the battery is sufficiently small (includingnear-zero currents) so that dynamic effects are negligible to allow OCVmeasurements to be taken within a desired accuracy. Thus, terms “restedOCV measurement” and “relaxed OCV measurement” within this disclosureare used interchangeably to mean an OCV measurement performed at theaforementioned “rested” or “relaxed” state of the battery. In thediscussion below, the term “OCV measurement” generally refers to theaforementioned “rested/relaxed OCV measurement,” unless the contextclearly dictates otherwise. Moreover, the term “resting voltage”throughout this disclosure refers to the voltage associated with the“rested/relaxed OCV measurement,” unless the context clearly dictatesotherwise.

Overview

The present disclosure provides a technique for accurately and reliablyupdating the full charge capacity (also referred to as “Q_(max)” below)of a battery. The aforementioned problem with infrequent updates of thefull charge capacity commonly results from users not letting theirsystems relax at low states of charge. The proposed solution to thisproblem involves modifying the charging process by inserting a restpoint during charging, thereby allowing a rested OCV measurement to beperformed. Note that battery cells can relax at very different rates atdifferent states of charge. To minimize the impact on the user from therest point measurement, the rest point can be carefully chosen to be ata state of charge when the cells comparatively quickly reach a steadystate where a measurement can be acquired. To further shorten the resttime needed, the charging current to the rest point may be controlled ata fixed level. This ensures that there is little or no dynamic behaviorduring the cell relaxation, thereby allowing the open circuit voltagemeasurement to be accelerated by extrapolating quickly to the restingvoltage. This controlled charging process is described in more detailbelow.

Coulomb Counting Based Q_(max) Measurement

FIG. 1 illustrates a coulomb counting based Q_(max) measurementtechnique for a battery in accordance with some embodiments herein. Asillustrated in FIG. 1, two rest measurements are performed on thebattery. The first rest measurement measures a first open circuitvoltage (OCV) value of the battery corresponding to anintermediate-level state-of-charge (SOC) “q_(prev),” wherein q_(prev)can be derived from the first OCV value based on a predetermined OCV vs.SOC relationship which has been calibrated for the battery. For example,the OCV vs. SOC relationship shown in FIG. 1 can be used. In the exampleshown, q_(prev) is between 50% and 75% SOC. In an ideal case, the firstrest measurement occurs when the battery is allowed to relax (i.e.,unplugged and unused) for a sufficiently long period of time until thatthe first OCV value can be reliably measured.

Also shown in FIG. 1, the second rest measurement measures a second OCVvalue of the battery at a full or near full SOC “q_(ful),” whereinq_(ful) can be derived from the second OCV value based on thepredetermined OCV-SOC relationship. In the example shown, q_(prev) is at100% SOC. Typically, the second rest measurement occurs when the batteryreaches the full SOC and the system remains plugged in for asufficiently long period of time until the second OCV value can bereliably measured.

Separately, a coulomb counting is performed during a charging processfrom the lower SOC “q_(prev)” to the full SOC “q_(ful)” to determine theamount of charge flow ΔQ during charging. Note that the system can useany current-sensing-based coulomb counting techniques to obtain ΔQ. Inone embodiment, the ΔQ measurement takes place between the two restmeasurements. In some embodiments, the system measures ΔQ prior to orafter both q_(prev) and q_(ful) have been determined. Finally, thesystem determines the full charge capacity Q_(max) for the battery basedon q_(prev), q_(ful), and ΔQ. In one embodiment, Q_(max) is computed by:

$Q_{\max} = {\frac{\Delta \; Q}{q_{ful} - q_{prev}}.}$

In practice, a user rarely rests long enough at low states of charge toallow q_(prev) and ΔQ to be measured. As a result, the conventionalQ_(max) updates based on the above-described approach take place veryinfrequently. This problem gets even worse on devices with backgroundservices that constantly disturb resting, thus preventing the batteryfrom reaching a relaxed state.

A Modified Charging Operation with Inserted Rest Points

Conventional charging of a battery does not include controlledinterruptions in the course of the charging process until the battery isunplugged from the charging source. Proposed embodiments modify theconventional charging process by inserting at least one rest pointduring the charging process which temporarily interrupts the chargingcurrent to allow the battery to relax, thereby allowing a rested OCVmeasurement to be performed during the interruption of the chargingprocess. In one embodiment, the at least one rest point is associatedwith a predetermined state of charge. We refer to this modified chargingprocess which includes at least one inserted rest point prior toreaching a fully charged state as a “controlled Q_(max) update.” FIG. 2presents a flowchart illustrating a process of charging a battery thatincludes a controlled Q_(max) update in accordance with some embodimentsherein.

Typically, the process starts when the system detects that the system isplugged in, for example, when the user plugs in the system to a chargeror a power source (step 202). Note that the system described herein mayinclude a battery management unit (BMU). The system then determines ifit is necessary to perform a controlled Q_(max) update (step 204). Inone embodiment, the system determines if it is necessary to perform acontrolled Q_(max) update based on a predicted uncertainty associatedwith Q_(max). More specifically, the system compares the uncertainty ofthe most recently updated Q_(max) with a threshold uncertainty. In thiscase, the system triggers a controlled Q_(max) update if the Q_(max)uncertainty exceeds the threshold uncertainty. On the other hand, thesystem bypasses the controlled Q_(max) update if the Q_(max) uncertaintyis below the threshold uncertainty. Note that Q_(max) uncertaintygenerally increases with time. Hence, the system can determine if acontrolled Q_(max) update is necessary based on how much time haselapsed since the last Q_(max) update has taken place. In oneembodiment, the system triggers a controlled Q_(max) update when apredetermined time period (e.g., one month) since the last Q_(max)update has been reached.

If it is determined that performing a controlled Q_(max) update is notnecessary, the system proceeds to conventionally charging the battery tofull, without stopping at a rest point during the charging process (step210). However, if it is determined that performing a controlled Q_(max)update is necessary, the system additionally determines if performing acontrolled Q_(max) update is possible (step 206). In some embodiments,performing step 204 is optional and the system directly proceeds to step206 from step 202.

In one embodiment, determining if performing a controlled Q_(max) updateis possible involves determining if the initial state of charge of thebattery is below the state of charge of the battery associated with apredetermined rest point (also referred to as the “target state ofcharge” below). Note that one requirement of a controlled Q_(max) updateis that the system stops at the predetermined rest point correspondingto a higher state of charge (relative to the initial state of charge)during the charging process. Consequently, one prerequisite associatedwith the controlled Q_(max) update is that the battery has discharged toa state of charge below the target state of charge. For example, if theinitial state of charge is 50% whereas the target state of charge is60%, the controlled Q_(max) update is deemed possible. On the otherhand, if the system determines that the initial state of charge of thebattery is above the target state of charge, the controlled Q_(max)update is deemed not possible.

If a controlled Q_(max) update is deemed not possible at step 206, thesystem proceeds to step 210 to conventionally charge the battery tofull, without performing the controlled Q_(max) update. On the otherhand, if a controlled Q_(max) update is deemed possible at step 206, thesystem proceeds to charge the battery through a modified chargingprocess with the controlled Q_(max) update (by inserting a rest point)(step 208). Note that the controlled Q_(max) update typically includesthe steps of controlled charging to a predetermined rest point (e.g., apredetermined cell voltage), relaxing at the rest point, and performinga rest measurement at the rest point. A detailed embodiment of thecontrolled Q_(max) update is described in conjunction with FIG. 3. Aftertaking the rest measurement at the rest point, the system proceeds tostep 210 to conventionally charge the battery to full.

After the system conventionally charges the battery to full and asufficient rest period has been reached (e.g., when the system remainsplugged in for a while), the system obtains an OCV measurement at thefully charged state of the battery (step 212). Note that step 212 mayfail to obtain an OCV measurement if the rest period is too short toallow the rest measurement to take place. For example, the user mayunplug the external power and start using the battery right away.

Next, the system determines if an OCV measurement at a rest point hasbeen taken during the charging process (step 214). As mentioned above,the system can reach step 214 without going through the controlledQ_(max) update (step 208) which obtains the OCV measurement at the restpoint. If it is determined that the OCV measurement has been taken atthe rest point, the system additionally determines if an OCV measurementat the fully charged state has been taken (step 216). If so, the systemproceeds to compute an updated Q_(max) using a coulomb counting basedQ_(max) update technique or other Q_(max) update techniques (step 218).In this case, the system obtains an updated Q_(max) through thecontrolled Q_(max) update. However, if either the OCV measurement at therest point or the OCV measurement at the fully charged state does notoccur, the charging process completes without a Q_(max) update.

Note that the above-described Q_(max) update process assumes that thefirst rest measurement for the Q_(max) update is taken at the rest pointand the second rest measurement is taken at the fully charged state. Ina variation to this embodiment, the system inserts two predeterminedrest points during the charging process between the initial state ofcharge and the full state of charge, and obtains a relaxed OCVmeasurement at each of the two rest points. In this embodiment, if therelaxed OCV measurement at the fully charged state fails to occur, thesystem can use both relaxed OCV measurements from the two rest points tocompute the updated Q_(max). However, if the relaxed OCV measurement atthe fully charged state is also taken, the system can choose one of thetwo OCV measurements from the two rest points and the OCV measurement atthe fully charged state to compute the updated Q_(max).

FIG. 3 presents a flowchart illustrating a modified charging processwhich uses an inserted rest point for a controlled Q_(max) update inaccordance with some embodiments herein. Note that FIG. 3 provides adetailed description of step 208 in FIG. 2. The embodiment assumes thata rest point is predetermined prior to the charging process. A moredetailed process for determining the rest point is provided below. Inone embodiment, the rest point corresponds to a predetermined state ofcharge of the battery (i.e., the “target state of charge”).

The process typically starts by conventionally charging the battery witha constant current from the initial state of charge toward the restpoint associated with the target state of charge (step 302). During thisprocess, the system constantly evaluates and compares the current stateof charge with the target state of charge of the rest point. Note thatbetween the initial state of charge and the target state of charge, thesystem can use more than one level of constant charging current. Forexample, FIG. 4 illustrates a charging profile 400 that employs aconstant-current constant-voltage charging profile in accordance withsome embodiments herein. More specifically, the system first applies afirst constant current 402 to the battery in the first constant currentregion 404 until a first target voltage is reached (e.g., at 4.0V).Next, the system applies the first constant voltage to the battery untilthe charging current reduces to a second constant current 406. Thesystem then applies the second constant current 406 to the battery inthe second constant current region 408 until a second target voltage isreached (e.g., at 4.1V). The system then applies the second constantvoltage to the battery until the charging current reduces to a thirdconstant current 410. Next, the system applies the third constantcurrent 410 to the battery in the third constant current region 412until a third target voltage is reached. The system then applies thethird constant voltage to the battery until the full state of charge isreached, and the charging is completed. Note that the embodiment showndoes not include a rest point for the controlled Q_(max) update.

Referring back to FIG. 3, when the target state of charge, i.e., restpoint is reached, the system interrupts the charging process byimmediately stopping the charging current and allowing the voltage torelax until an OCV measurement can be acquired (step 304). In someembodiments, instead of immediately dropping the charging current tozero and then resting, the system first reduces the charging current toa significantly low level above zero. This process may take a shortperiod of time, e.g., a few minutes. The system then starts resting thebattery by stopping the remaining charging current.

After the battery has sufficiently relaxed, the system obtains a relaxedOCV measurement at the rest point, thereby obtaining the restmeasurement at the target state of charge (step 306). Next, the systemresumes charging the battery (step 308) and conventionally charging thebattery to the full state of charge (i.e., continuing to step 210 inFIG. 2 of the overall charging process). Note that, if more than onerest point is inserted during the controlled Q_(max) update, the systemsimply repeats steps 302-308 each time a new rest point is reached.

FIG. 5 illustrates a charging profile 500 of a controlled Q_(max) updatethat includes a single rest point in accordance with some embodimentsherein. Note that the charging profile 500 with the controlled Q_(max)update illustrated in FIG. 5 is a variation of charging profile 400 ofFIG. 4, which is also shown in FIG. 5 for comparison. More specifically,charging profile 500 with the controlled Q_(max) update follows theoriginal charging profile 400 (shown in solid lines) initially. After apredetermined rest point at a time mark 502 (near the 150-minute mark),charging profile 500 enters the controlled Q_(max) update phase which isshown in dotted lines. As is illustrated in FIG. 5, the systemassociated with charging profile 500 drops the charging current from thethird constant current at 0.4 C-rate to zero. The system then waits forthe battery to rest during a rest period marked as 504. Next, the systemtakes the rest measurement at time mark 506. The system then resumes thecharging process by restoring to the third constant current andcontinues to charge the battery to full. Note that the overall chargingtime increase 508 from the original charging profile 400 as a result ofthe controlled Q_(max) update is substantially the same as the restperiod 504.

Note that, while more than one level of charging current may be used tocharge up toward the target state of charge, it is desirable to have aconstant charging current during a time period immediately preceding therest point. This ensures that there is little or no dynamic behaviorduring the cell relaxation, thereby allowing the open circuit voltagemeasurement be accelerated by extrapolating quickly to the restingvoltage.

Note that the controlled Q_(max) update profile is substantially similarto the original charging profile of FIG. 4 except for additional restperiod 504 occurring between rest point 502 and rest measurement point506. When the rest point is chosen properly, the charging time increasesuch as time 508 is insignificant and will not degrade the userexperience. However, if the rest point is chosen improperly, thecharging time increase due to the rest measurement may be significantcompared with the regular charging time and can negatively affect theuser experience.

Choosing Rest Points

As mentioned above, the rest point associated with the controlledQ_(max) update can be predetermined prior to the charging process. Whenchoosing a rest point for the controlled Q_(max) update, a number ofinterrelated considerations are to be balanced. These considerations caninclude, but are not limited to, the time required for the battery torelax at the rest point; the uncertainty associated with coulomb countΔQ from the rest point to the full charge capacity; the likelihood ofthe user discharging below the rest point; and the voltage and curveaccuracy required to establish the state of charge at a given restpoint.

Among the above considerations, the time required for the battery torelax is one of the more important considerations because this timeaccounts for an increased charging time for the system, which cannegatively impact the user experience. Hence, a rest point associatedwith a fast time to relax is preferred. The time needed for a cell torelax is state-of-charge dependent and needs to be characterized on aper cell-chemistry basis. For example, at 60% SOC it can take one hourto relax, but at 80% SOC it may take only minutes to relax. FIG. 6illustrates an exemplary cell relaxing profile 600 after a constantcurrent charging process has been applied up to a rest point at a highSOC in accordance with some embodiments herein. The data shows that thetime to relax, or the additional charging time, can be kept to a fewminutes, depending on how large an error in full charge capacity can betolerated. If a rest point is chosen based on this profile, it isreasonable to rest for somewhere between three or four minutes to get anaccurate open circuit voltage measurement. While charging a battery tofull capacity usually takes on the order of hours, the additionalminutes of charging time are probably insignificant to the userexperience.

While it is desirable to choose the rest point at a higher SOC based onthe fast-to-relax consideration, a rest point at a very high SOC cancause a decrease in accuracy in determining the Q_(max). In theabove-described coulomb counting technique, Q_(max) is determined bytaking the difference between the two open circuit voltage measurementsand dividing by the coulomb count ΔQ, which itself is determined duringthe charging process between the two rest measurements. Because thecoulomb count has an error associated with it, if the SOC at the restpoint is too high, the measured ΔQ is small and the uncertainty in themeasurement increases. This uncertainty in ΔQ is then propagated to theuncertainty in Q_(max). For this consideration alone, a lower SOC isdesirable for the measurement accuracy, but it could be in conflict withthe fast-to-relax consideration. When balancing the two considerations,the rest point should be placed above a given low state of charge andbelow a given high state of charge. In one embodiment, the rest point isbetween 60% and 80% SOC to balance these two considerations. Below 60%SOC, relaxation times are significantly increased, whereas above 80%SOC, the error from the coulomb count measurement starts to getnoticeable.

Note that, in order for the controlled Q_(max) update to occur, the userhas to discharge below the predetermined rest point. In systems with avery large capacity battery, such as the iPad, discharging below a highSOC rest point can take a significant amount of time. Hence, thelikelihood of the user discharging below a given SOC associated with apotential rest point has to be balanced against the time required forthe battery to relax.

Another factor which needs to be taken into account in determining therest point is related to the OCV measurement voltage and curve accuracyrequired to establish state of charge at a given rest point. Referringto FIG. 1, note that the OCV-SOC curve includes linear regions andnon-linear regions. If the rest point is chosen right between tworelatively linear regions shown as “X”, the SOC accuracy is likely tosuffer. However, if the rest point is chosen at the q_(prev) in therelatively linear region, the SOC accuracy may be improved.

In addition to the above considerations, other factors, such astemperature and charging/discharging history of the system may be takeninto account and balanced with all other considerations in determining aproper rest point. FIG. 7 provides a chart illustrating different stateof charge regions which may be used to guide the choice of a proper restpoint. Note that the chart shows that the optimal rest point region isapproximately between 60% and 80% SOC.

Battery Design

FIG. 8 illustrates a rechargeable battery 800 that supports a controlledQ_(max) update during charging in accordance with some embodimentsherein. Battery 800 includes a battery cell 802. It also includes acurrent meter (current sensor) 804, which measures a charging currentthrough cell 802, and a voltmeter (voltage sensor) 806, which measures avoltage across cell 802. Battery 800 also includes a thermal sensor 830,which measures the temperature of battery cell 802. (Note that numerouspossible designs for current meters, voltmeters and thermal sensors arewell-known in the art.)

Rechargeable battery 800 is coupled to a charger 823. Note that,although charger 823 is illustrated in FIG. 8, charger 823 is typicallynot part of rechargeable battery 800. However, charger 823 is associatedwith the charging process by providing charging current to cell 802.Note that charger 823 can include any battery charging mechanism nowknown or later developed. This includes, but is not limited to, a USBcharger, a multi-pin charger, a docking station, a portable charger, anda wall charger.

The above-described modified charging process with a controlled Q_(max)update is controlled by a controller 820, which receives: a voltagesignal 808 from voltmeter 806, a current signal 810 from current meter804, a temperature signal 832 from thermal sensor 830, and a state ofcharge (SOC) value 834 from SOC estimator 833. Additionally, controller820 stores one or more predetermined state of charge values 836. Thesestate of charge values are used to generate one or more rest pointsduring the controlled Q_(max) update. Controller 820 can include acoulomb counter 838 for estimating the amount of charge flow ΔQ based oncurrent 810 during a charging process. Controller 820 can also generatecontrol signals 840 for controlling a charging current of charger 823.Control signals 840 can also control a switch 842. In some embodiments,control signals 840 can be used to turn off switch 842 to decouplecharger 823 from cell 802.

During a charging operation, controller 820 controls SOC estimator 833to determine two SOC values corresponding to the two relaxed OCVmeasurements at a rest point and at the full charge. SOC estimator 833receives a voltage 808 from voltmeter 806, a current 810 from currentmeter 804 and a temperature 832 from thermal sensor 830, and outputs astate of charge value 834. Controller 820 outputs a full charge capacityestimate 844 of cell 802 based on two SOC values corresponding to thetwo relaxed OCV measurements and an estimated coulomb count between thetwo relaxed OCV measurements.

Note that controller 820 can be implemented using either a combinationof hardware and software or purely hardware. In one embodiment,controller 820 is implemented using a microcontroller, which includes amicroprocessor that executes instructions which control the full chargecapacity update process.

The foregoing descriptions of embodiments have been presented forpurposes of illustration and description only. They are not intended tobe exhaustive or to limit the present description to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present description. The scopeof the present description is defined by the appended claims.

What is claimed is:
 1. A method for accurately estimating a capacity ofa battery, comprising: charging the battery from an initial state to arest point prior to reaching a fully charged state; interrupting thecharging process to allow the battery to relax to a resting voltage;measuring the resting voltage; resuming the charging process toward thefully charged state; and estimating the capacity of the battery based atleast on the resting voltage.
 2. The method of claim 1, wherein prior tocharging the battery, the method further comprises: determining if astate of charge of the battery corresponding to the initial state isabove a state of charge of the battery corresponding to the rest point;and if so, charging the battery from the initial state to the fullycharged state without interrupting the charging process.
 3. The methodof claim 1, wherein the capacity is a full charge capacity.
 4. Themethod of claim 1, wherein charging the battery from the initial stateto the rest point involves charging the battery with a constant currentduring a time period immediately preceding the rest point.
 5. The methodof claim 4, wherein interrupting the charging process involvesimmediately dropping the charging current to zero.
 6. The method ofclaim 4, wherein interrupting the charging process involves reducing thecharging current to a low level above zero.
 7. The method of claim 1,wherein after reaching the fully charged state, the method furthercomprises: allowing the battery to relax to a second resting voltage;and measuring the second resting voltage corresponding to the fullycharged state.
 8. The method of claim 7, wherein determining thecapacity of the battery based at least on the resting voltage involves:computing a first state of charge of the battery corresponding to theresting voltage; computing a second state of charge of the batterycorresponding to the second resting voltage; determining a coulomb countbetween the rest point and the fully charged state; and determining thecapacity of the battery based on the first state of charge, the secondstate of charge, and the coulomb count.
 9. The method of claim 8,wherein determining the coulomb count between the rest point and thefully charged state involves: initiating a coulomb counting from therest point after resuming the charging process; concluding the coulombcounting when the fully charged state is reached; and determining thecoulomb count based on the coulomb counting between the rest point andthe fully charged state.
 10. The method of claim 8, wherein each of theresting voltage and the second resting voltage is an open circuitvoltage (OCV).
 11. The method of claim 1, wherein prior to charging thebattery, the method further comprises selecting the rest point based atleast on one or more of the following parameters: a time required forthe battery to relax at the rest point; an error associated with acoulomb counting between the rest point and the fully charged state; alikelihood of a user discharging the battery below the rest point; andan open-circuit-voltage measurement accuracy required to compute a stateof charge associated with the rest point.
 12. The method of claim 11,wherein selecting the rest point involves ensuring that the timerequired for the battery to relax at the rest point is significantlyshorter than a time to charge the battery from the initial state to thefully charged state without interruption.
 13. The method of claim 11,wherein the rest point is in the vicinity of or above a 60% state ofcharge of the battery.
 14. The method of claim 11, wherein the restpoint is in the vicinity of or below an 80% state of charge of thebattery.
 15. A computer-readable storage medium storing instructionsthat when executed by a controller for a battery cause the controller toperform a method for accurately estimating a capacity of the battery,the method comprising: charging the battery from an initial state to arest point prior to reaching a fully charged state; interrupting thecharging process to allow the battery to relax to a resting voltage;measuring the resting voltage; resuming the charging process toward thefully charged state; and estimating the capacity of the battery based atleast on the resting voltage.
 16. The computer-readable storage mediumof claim 15, wherein prior to charging the battery, the method furthercomprises: determining if a state of charge of the battery correspondingto the initial state is above a state of charge of the batterycorresponding to the rest point; and if so, charging the battery fromthe initial state to the fully charged state without interrupting thecharging process.
 17. The computer-readable storage medium of claim 15,wherein the capacity is a full charge capacity.
 18. Thecomputer-readable storage medium of claim 15, wherein charging thebattery from the initial state to the rest point involves charging thebattery with a constant current during a time period immediatelypreceding the rest point.
 19. The method of claim 18, whereininterrupting the charging process involves immediately dropping thecharging current to zero.
 20. The computer-readable storage medium ofclaim 15, wherein after reaching the fully charged state, the methodfurther comprises: allowing the battery to relax to a second restingvoltage; and measuring the second resting voltage corresponding to thefully charged state.
 21. The computer-readable storage medium of claim20, wherein determining the capacity of the battery based at least onthe resting voltage involves: computing a first state of charge of thebattery corresponding to the resting voltage; computing a second stateof charge of the battery corresponding to the second resting voltage;determining a coulomb count between the rest point and the fully chargedstate; and determining the capacity of the battery based on the firststate of charge, the second state of charge, and the coulomb count. 22.The computer-readable storage medium of claim 21, wherein determiningthe coulomb count between the rest point and the fully charged stateinvolves: initiating a coulomb counting from the rest point afterresuming the charging process; concluding the coulomb counting when thefully charged state is reached; and determining the coulomb count basedon the coulomb counting between the rest point and the fully chargedstate
 23. The computer-readable storage medium of claim 15, whereinprior to charging the battery, the method further comprises selectingthe rest point based at least on one or more of the followingparameters: a time required for the battery to relax at the rest point;an error associated with a coulomb counting between the rest point andthe fully charged state; a likelihood of a user discharging the batterybelow the rest point; and an open-circuit-voltage measurement accuracyrequired to compute a state of charge associated with the rest point.24. The computer-readable storage medium of claim 23, wherein selectingthe rest point involves ensuring that the time required for the batteryto relax at the rest point is significantly shorter than a time tocharge the battery from the initial state to the fully charged statewithout interruption.
 25. A battery with a capacity estimationmechanism, comprising: a cell; a voltage sensor configured to measure avoltage for the battery; a current sensor configured to measure acurrent for the battery; and a controller configured to receive inputsfrom the voltage sensor and the current sensor, and to generate acapacity estimate; wherein the controller is configured to: charge thebattery from an initial state to a rest point prior to reaching a fullycharged state; interrupt the charging process to allow the battery torelax to a resting voltage; measure the resting voltage; resume thecharging process toward the fully charged state; and estimate thecapacity of the battery based at least on the resting voltage.
 26. Thebattery of claim 25, wherein prior to charging the battery, thecontroller is configured to: determine if a state of charge of thebattery corresponding to the initial state is above a state of charge ofthe battery corresponding to the rest point; and if so, charge thebattery from the initial state to the fully charged state withoutinterrupting the charging process.
 27. The battery of claim 25, whereinthe controller is configured to charge the battery from the initialstate to the rest point by charging the battery with a constant currentduring a time period immediately preceding the rest point.
 28. Thebattery of claim 27, wherein the controller is configured to interruptthe charging process by immediately dropping the charging current tozero.
 29. The battery of claim 25, wherein after reaching the fullycharged state, the controller is configured to: allow the battery torelax to a second resting voltage; and measure the second restingvoltage corresponding to the fully charged state.
 30. The battery ofclaim 29, wherein the controller is configured to determine the capacityof the battery by: computing a first state of charge of the batterycorresponding to the resting voltage; computing a second state of chargeof the battery corresponding to the second resting voltage; determininga coulomb count between the rest point and the fully charged state; anddetermining the capacity of the battery based on the first state ofcharge, the second state of charge, and the coulomb count.
 31. Thebattery of claim 30, wherein the controller is configured to determinethe coulomb count between the rest point and the fully charged state by:initiating a coulomb counting from the rest point after resuming thecharging process; concluding the coulomb counting when the fully chargedstate is reached; and determining the coulomb count based on the coulombcounting between the rest point and the fully charged state.
 32. Amethod for evaluating a state of charge of a battery during a chargingprocess, the method comprising: charging the battery from an initialstate to a rest point prior to reaching a fully charged state;interrupting the charging process to allow the battery to relax to aresting voltage; measuring the resting voltage; and determining a stateof charge of the battery at the rest point based at least on the restingvoltage.